So why is this not assuming the reaction has gone to completion? Surely the reaction must reach completion to become basic?
Look at the HH equation and think what will happen with pKa=7 acid
The stronger the acid, the earlier the solution becomes basic. To take extreme example - hydrocyanic acid, pKa = 9.31
0.01M solution has pH 5.65 - slightly acidic.
if you add 0.5% (!) of strong base, neutralizing 0.5% HCN, pH becomes 7.01